The present invention relates to a method of enhancing residual compressive stress in double metallic tubes.
It is known that in metallic materials, for example, austenitic stainless steels which are widely used in nuclear powers and chemical plants, corrosion cracking rapidly proceeds when there exist both tensile stress and a corrosion factor. There has been proposed a method for overcoming such a defect in a metallic pipe by providing a residual compressive stress in it. In this method, the metallic pipe is induction heated for providing the temperature difference between its inner and outer faces to generate thermal stress larger than its yield point, while cooling water passes through it. This results in a residual compressive stress in a radially inner portion of a welded portion, such as a joint, of the metallic pipe.
This prior art method may be applied to metallic pipes having simple configurations such as straight pipe. In double metallic pipes, for example, a double metallic pipe having a thermal sleeve concentrically fitted into it, it is, however, very hard to provide the temperature difference to generate sufficient residual compressive stress in it since circulation of cooling water is prevented due to residence of the cooling water between the inner and outer pipes.
Accordingly, it is an object of the present invention to provide a method of enhancing residual compressive stress in a double metallic tube, in which a necessary radial temperature difference between the inner and outer face of the outer pipe is provided with ease for generating sufficient residual compressive stress.